Conventional Schottky diodes have the characteristic that, when they are loaded with a voltage in the reverse direction, the greatest electrical field strength occurs directly adjacent to the surface of the Schottky contact. This means that a considerably increased leakage current occurs in Schottky diodes even considerably below the theoretical breakdown field strength of its semiconductor material. In addition to silicon, silicon carbide (SiC), for example, may also be used as the semiconductor material.
As a result of the above characteristic, Schottky diodes are in practice designed such that the electrical field strength in them remains considerably below the theoretically achievable breakdown field strength even at the rated voltage. This reduced electrical field strength at the rated voltage may be achieved by means of a reduced dopant concentration in the semiconductor body of the Schottky diode and/or by designing the drift path to be longer, that is to say by designing the semiconductor body to be thicker, and/or by the use of a Schottky contact with a high energy barrier for the charge carriers. Thus, for example, U.S. Pat. No. 5,262,668 describes a Schottky diode with areas having different energy barriers.
All of these measures for reducing the electrical field strength at the rated voltage lead, however, to the forward voltage of the Schottky diode being considerably greater than it would need to be without the measures taken to overcome the surface problems explained above.
A further problem, in particular in the case of silicon carbide Schottky diodes, is their relatively low resistance to overcurrents. The forward voltage of silicon carbide Schottky diodes is approximately proportional to T2.5 (T: Temperature), so that considerable values are reached even at room temperature. In numerous applications, for example for PFC (Power Factor Correction) purposes, however, better resistance to overcurrents is, however, required in some operating states. Such a greater resistance to overcurrents would make it possible to use smaller silicon carbide Schottky diodes. This is because, at the moment, the Schottky diodes are often chosen on the basis of the resistance to overcurrents required for their application, and they are thus derated.
It is now known from “Power Semiconductor Devices” by B. Jayant Baliga, PWS Publishing, 1995, pages 182 to 195 that, in the case of silicon Schottky diodes with an n-doped drift path, the surface field strength adjacent to the Schottky contact can be reduced considerably, when it is reverse-biased, by introducing p-doped regions on the surface of one face of the Schottky diode and with the distances between these regions being suitably matched, that is to say the p-doped regions are located in the n-doped area. Such p-doped regions have already been proposed, for example, as raised islands (see U.S. Pat. No. 4,982,260), as a combination of pn junctions and Schottky junctions (see U.S. Pat. No. 5,101,244) and as doping on side walls and at the bottom of a trench (see U.S. Pat. No. 5,241,195).
Furthermore, pinch structures with highly doped n-conductive regions (U.S. Pat. No. 4,641,174) and compensation structures with p-doped columns (DE 197 40 195 C2) have also already been described for Schottky diodes in a semiconductor body.
The incorporation of such p-doped regions in a rectangular grid has also already been generally considered in conjunction with silicon carbide Schottky diodes (see, in this context, the article “Comparison of 4H SiC pn, Pinch and Schottky Diodes for the 3 kv Range” by Peters, Friedrichs, Schörner, Stephani in Materials Science Forum, Volume 389-393, pages 1125-1128). The reduction in the electrical field strength immediately adjacent to the Schottky contact area as a consequence of the incorporation of p-doped regions in the otherwise n-doped semiconductor body means that the Schottky diodes may be designed to have higher doping, which reduces the forward voltage.
An additional advantage of these p-doped regions is that they are suitably dimensioned such that, when the current density of the current flowing through the Schottky diode is relatively high, the p-doped regions inject charge carriers, and thus ensure that the voltage drop is considerably less. The current density for this “starting” of the injection process is in general designed to be sufficiently high that the diode operates only as a Schottky diode at the rated current, and that bipolar conduction resulting from the injection process occurs only in the event of overcurrents, for example at twice the rated current.
The starting current for the bipolar injection process is, to a first approximation, proportional to the n-dopant concentration in the semiconductor body, and is inversely proportional to the minimum distance between the center of the p-doped region and the closest n-doped region, that is to say is inversely proportional to the minimum extent of the p-doped regions.
This means that, if the n-dopant concentrations are relatively high, the p-doped regions must be made broader in order to reduce the forward voltage. Specifically, in order to achieve a shielding effect for the electrical field on the surface, the distance between the p-doped regions must be reduced correspondingly if the n-dopant concentration is relatively high, in order to achieve the desired effect at all points on the Schottky content.
The higher dopant concentration in the n-doped semiconductor body thus means that the p-doped regions must be larger and must be placed closer together. This in turn leads to a considerable loss of surface area for the active Schottky part of the diode, and thus to a major rise in the forward voltage.
Thus, overall, it is virtually impossible for relatively small reverse voltages to find suitable dimensions for Schottky diodes, which at the same time also produce a low forward voltage while achieving a good shielding effect and good resistance to overcurrents. No solution to this problem has been found until now, or has even been considered to be possible, so that the problem has been accepted as such.